Polyimide resins are widely used in the fields of electrical and electronic materials, adhesives, coatings, composite materials, and fiber and film materials, because they are excellent in heat resistance, wear resistance, chemical resistance, electrical insulating properties and mechanical characteristics.
Polyimide resin films are also used in various ways as coating materials for electric wires, cables and wires and as insulating materials for transformers and printed-circuit boards, because of their excellent characteristcs.
Many patents exist which disclose a process for the production of a polyimide resin from a polyamic acid as a precursor in which the polyamic acid is prepared by a polyaddition reaction of a tetracarboxylic acid dianhydride and an aromatic diamine in an organic solvent and then subjected to dehydration and cyclization by means of heating or a chemical dehydrating using a dehydrating agent.
In general, a polyamic acid as a precursor of a polyimide resin is prepared by means of polyaddition of a tetracarboxylic acid dianhydride and an aromatic diamine in an organic solvent with a polymer concentration of 5 to 20% by weight to obtain a high molecular weight polyamic acid homogeneously dissolved in the organic solvent. A film or a molded material is then prepared by removing the organic solvent from the thus prepared polyamic acid solution. Thereafter, the film or molded material is subjected to dehydration and cyclization by means of a high temperature treatment or a chemical treatment to obtain a molded polyimide material.
A simplified process has also been disclosed in, for example, JP-A-57-109614 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") in which a polyamic acid dissolved in an organic solvent is mixed with a dehydrating agent and a catalyst and then the mixture is dried to obtain a polyimide film directly from the thus formed polyamic acid gel film.
With regard to a blend of a polyimide with another high polymer component, a composition in which a polyimide was mixed with a thermoplastic resin such as a polysulfone for the purpose of improving moldability has been disclosed, for example, in JP-A-63-304054 and JP-A-63-305166.